Spotlight on Circle Equations Skills you will need
Description: Spotlight on Circle Equations Skills you will need You will need to be able to apply Pythagoras theorem know that a tangent is perpendicular to a radius at the point where they meet use the equation of a circle centred at the origin WJEC
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slide1. Spotlight on
Circle Equations<br>
slide2. Skills you will need You will need to be able to
apply Pythagoras’ theorem
know that a tangent is perpendicular to a radius at the point where they meet
use the equation of a circle centred at the origin © WJEC CBAC Ltd.<br>
slide3. Understanding Check 
P(x, y) x y 10 © WJEC CBAC Ltd.<br>
slide4. Summer Series 2017, C1H Q17 Common misunderstanding:
As the chord AB is 24, the gap between the circles is 24  4 = 6 cm , is that correct?
No! This diagram is not drawn to scale.
It may look approximately like this gap is 6 cm but in fact it is not and this is an assumption made most likely from measuring the diagram.
Diagram not drawn to scale means that you must not measure the diagram. 6 cm © WJEC CBAC Ltd.<br>
slide5. What the examiner said © WJEC CBAC Ltd.<br>
slide6. Summer Series 2017, C1H Q17 Step 1: Given knowledge
The chord AB is 24 cm.
, so the radius of the outer circle is 13 cm.
Step 2: Required knowledge
The equation of the inner circle which has the form
Find the radius of the inner circle. R 24 © WJEC CBAC Ltd. 13 13 12<br>
slide7. What the examiner said ...
Some candidates indicated that the radius of the outer circle was 13 and others showed that the half chord was important.
A few candidates listed square numbers and demonstrated that 52 + 122 = 132 , but were unable to do anything further with this information. © WJEC CBAC Ltd.<br>
slide8. Summer Series 2017, C1H Q17 Step 3:
Apply Pythagoras’ theorem:
Step 4: The equation of the inner circle is R © WJEC CBAC Ltd. 13 12<br>
slide9. End<br>
Circle Equations<br>
slide2. Skills you will need You will need to be able to
apply Pythagoras’ theorem
know that a tangent is perpendicular to a radius at the point where they meet
use the equation of a circle centred at the origin © WJEC CBAC Ltd.<br>
slide3. Understanding Check 
P(x, y) x y 10 © WJEC CBAC Ltd.<br>
slide4. Summer Series 2017, C1H Q17 Common misunderstanding:
As the chord AB is 24, the gap between the circles is 24  4 = 6 cm , is that correct?
No! This diagram is not drawn to scale.
It may look approximately like this gap is 6 cm but in fact it is not and this is an assumption made most likely from measuring the diagram.
Diagram not drawn to scale means that you must not measure the diagram. 6 cm © WJEC CBAC Ltd.<br>
slide5. What the examiner said © WJEC CBAC Ltd.<br>
slide6. Summer Series 2017, C1H Q17 Step 1: Given knowledge
The chord AB is 24 cm.
, so the radius of the outer circle is 13 cm.
Step 2: Required knowledge
The equation of the inner circle which has the form
Find the radius of the inner circle. R 24 © WJEC CBAC Ltd. 13 13 12<br>
slide7. What the examiner said ...
Some candidates indicated that the radius of the outer circle was 13 and others showed that the half chord was important.
A few candidates listed square numbers and demonstrated that 52 + 122 = 132 , but were unable to do anything further with this information. © WJEC CBAC Ltd.<br>
slide8. Summer Series 2017, C1H Q17 Step 3:
Apply Pythagoras’ theorem:
Step 4: The equation of the inner circle is R © WJEC CBAC Ltd. 13 12<br>
slide9. End<br>